The NVS 420 might be a bit over kill for most applications. We had a similar issue at our school. Our Optiplex 980 was used to display some slides (nothing complex) as part of a digital signage system. We mirrored the displays out to 3 different monitors using a VGA splitter. Similar to the ones found here :

If you just need one more display for workstation usage, a USB to (VGA) or (DVI) adapter might also work for you. It won't do high definition video rendering, but most desktop applications will work fine.